If you never heard of Paul Graham, you should check out his web site and read couple of his essays. He has got some great ideas and insights. For example this one, The Top Idea In Your Mind is just a short few paragraphs, but packs a powerful idea. Here is a except:
“…I'd say it's hard to do a really good job on anything you don't think about in the shower.”…
“I think most people have one top idea in their mind at any given time. That's the idea their thoughts will drift toward when they're allowed to drift freely. And this idea will thus tend to get all the benefit of that type of thinking, while others are starved of it. Which means it's a disaster to let the wrong idea become the top one in your mind.”
The key is to influence what the ‘top idea’ is that is rattling around your mind consuming the vast majority of your mental resources. You can not directly control this type of “ambient thought”. You can indirectly influence what you mind drifts to by emotionally feeding the ideas you want to grow and starving those you don’t. Things that you let become important to you; i.e. making money, the conquest of a lover, etc. will tend to dominate your ambient thoughts.